1. Field of the Invention
This invention pertains to the art of fluid conditioning apparatus, systems and methods, and is more particularly concerned with improvements for preventing excessive ice formation at the cold fluid inlet of heat exchangers in these systems, as well as for providing improved heating capacity utilizing the heat transfer performance capabilities of heat exchangers. These apparatus are most commonly used, but are not limited to, environmental control systems for air and ground vehicles, both military and commercial.
2. Description of Prior Art
Examples of arrangements for preventing excessive ice formation are described in three patents. The first is U.S. Pat. No. 4,198,830 dated Apr. 20, 1980, of Carl D. Campbell, and entitled "Fluid Conditioning System And Apparatus". The second is U.S. Pat. No. 4,246,963 dated Jan. 27, 1981, of Alexander Anderson, and entitled "Heat Exchanger". The third is U.S. Pat. No. 4,352,273 dated Oct. 5, 1982, of Robert C. Kinsell et al, and entitled "Fluid Conditioning Apparatus And System". All of the above patents are assigned to the same assignee as that of the present invention.
Briefly stated, the Campbell invention provides a means to condense water out of the working fluid of an air-cycle environmental control system while the fluid (air) was still at high pressure, eliminating the need for coalescer bags which require significant maintenance. A problem with ice formation on the face of the heat exchanger in line with the discharge of the expansion turbine was addressed by the Anderson invention which proposed the addition of hot header bars, not connected to the cold passages, for improved ice prevention characteristics of the heat exchanger. The Anderson invention also detailed the heat transfer relationship between the hot and the cold side fins necessary to maintain metal temperatures above freezing. The Kinsell et al invention provides for the substantial lowering of the expansion turbine discharge air temperature and an associated increase in the delivered cooling capacity of a typical apparatus by adding a bypass in the middle of the heat exchanger.
In addition to the type of air cycle environmental control system (ECS) described in those patents, the present invention is useful in an ECS system variation wherein condensing heat exchanger is used to achieve extra heating capacity in a combined ECS, nuclear-biological contaminant filtration system while maintaining the proper environmental conditions at the filter inlet. This cycle also relies on the three previously described patents for the prevention of ice in subfreezing condensers. The above inventions have found successful application in both commercial and military air and ground vehicle environmental control systems However, the methods described above for controlling the formation of ice have normally required additional, active methods of ice control.
Applications of the Anderson invention have shown that the performance of the condenser heat exchanger changes dramatically with changes in operating conditions, specifically turbine discharge velocities. This is due to the design of the heat exchanger cores described in the Anderson invention. The variation in performance is due to the very low pressure loss of the condenser core relative to the manifold pressure losses. In other heat exchangers, core pressure losses are typically 80% of the flange-to-flange pressure loss to ensure proper flow distribution through the heat exchanger. The Anderson invention provides for very loose fins on the cold side passage of the heat exchanger to prevent the accumulation of ice by reducing the blockage of the flow path and by biasing the metal temperatures closer to the hot side than the cold side. The resulting design is therefore very sensitive to flow stratification at the condenser cold side inlet.
Testing of various systems utilizing the referenced inventions has shown that, when the turbine is close-coupled to the condenser, as it is in most systems, changes in turbine exit velocity produce significant changes in the flow and temperature stratification of the condenser cold side inlet.